As an imaging apparatus used for a medical image diagnosis and a non-destructive inspection using radiation (X-rays), a radiation imaging apparatus using a flat panel detector (FPD) formed of semiconductor material has been used. Such a radiation imaging apparatus may be used as a digital imaging apparatus which captures still images and moving images in the medical image diagnosis, for example.
Examples of the FPD include an integral sensor and a photon counting sensor. The integral sensor measures a total amount of charge generated by incident radiation. The photon counting sensor discriminates energy (wavelengths) of incident radiation and counts the numbers of times the radiation is detected for individual energy levels. Specifically, since the photon counting sensor has energy resolution capability, the photon counting sensor is expected to be applied to discrimination of substances and generation of an image and measurement of bone density in a case where imaging is virtually performed with monoenergetic radiation. However, since the number of incident radiation quanta is large, a high operation speed is required for individually counting the radiation quanta. Accordingly, it is difficult to realize the photon counting sensor in an FPD having a large area.
Therefore, PTL 1 proposes a radiation imaging apparatus which realizes energy resolution capability by estimating the number of radiation quanta and an average value of energy using average image density information and distribution information of image density for each predetermined region. Specifically, PTL 1 discloses an information processing method for estimating the number of radiation quanta and an average value of the energy using average image density information and distribution information of image density for each predetermined region and obtaining two types of image information, that is, the number of radiation quanta and the average value of the energy of the radiation quanta. When the method disclosed in PTL 1 is employed, a sensor having energy resolution capability may be realized even in a case of a low operation speed when compared with the photon counting sensor.
On the other hand, PTL 2 discloses a technique of an energy subtraction method. When the energy subtraction method is employed, two images are obtained by irradiation of respective two types of energy and a difference process is performed on the two images which have been subjected to a desired calculation so that the images of two substances having different attenuation coefficients are generated in a discrimination manner. The energy subtraction method utilizes a phenomenon in which different substances have different attenuation coefficients indicating degrees of attenuation of radiation at times when the radiation passes through the substances and the attenuation coefficients depend on energy of the radiation. Furthermore, PTL 2 further discloses a technique of dual-energy X-ray absorptiometry method (a DEXA method) which is a technique of measuring bone density utilizing the same phenomenon. However, radiation imaging is performed twice using two types of energy in the energy subtraction method and the DEXA method disclosed in PTL 2. Therefore, there arise problems in that artifact is generated due to a movement of a subject while energy is switched and in that high speed switching of radiation energy is required. In terms of these problems, the processing method disclosed in PTL 1 is more advantageous since substances may be discriminated from each other by one radiation imaging using one type of energy.
However, PTL 1 does not disclose a method for obtaining information for discriminating two substances constituting a radiation image using two types of image information, that is, the obtained number of radiation quanta and the obtained average value of the energy the radiation quanta.